Preparation And Properties Of Ultra High Temperature Ceramic Precursors And Composites

Ultra high temperature materials with excellent mechanical and ablation properties were needed for the nose caps, sharp leading edges and rocket engines of hypersonic aerospace vehicles. The proposal to prepare continuous fiber reinforced ultra high temperature ceramics matrix composites by precursor infiltration and pyrolysis (PIP) process is presented to improve the properties of the existing ultra high temperature materials. TiC, ZrC and ZrB₂ precursors were firstly prepared by mixing-reaction method, then by use of these precursors C/ZrC and C/ZrC-SiC composites were fabricated, accordingly the influence of interface coatings on the properties of the composites, and the ablation mechanisms of the composites in arc-jet wind tunnel and oxyacetylene flame environments were investigated and discussed.TiC, ZrC and ZrB₂ precursors were prepared by mixing-reaction method, and the cross-linking and pyrolysis mechanisms were investigated. Firstly, TiC precursor was obtained by mixing titanium butoxide and divinylbenzene (DVB). The precursor cross-linked at 120²10oC via the cross-linking of DVB, and decomposed into t-TiO₂ and carbon at 283²97oC and 445⁴61oC, respectively. TiC was formed at 1200oC by carbo-thermal reaction. The crystalline degree of TiC increased while the content of DVB increased and/or pyrolysis temperature increased. Secondly, hydroxyl-ZrC precursor was prepared with zirconium butoxide and DVB as sources of zirconium and carbon, respectively. The precursor cross-linked at 120²10oC, and partialy decomposed into amorphous ZrO₂ at above 150oC. Amorphous ZrO₂ changed into t-ZrO₂ at 400oC. DVB decomposed into carbon at 400⁵00oC. ZrC was obtained by carbo-thermal reaction at temperature higher than 1400oC. The crystalline degree of ZrC increased while the content of DVB increased or the holding time at 1600oC increased. ZrC without oxide was obtained when the precursor with a Zr/C ratio of 1/3 was pyrolyzed at 1600oC for 2 hours. Thirdly, aqueous-ZrC precursor was prepared with zirconium oxychloride (ZrOCl2·8H2O) and sucrose as sources of zirconium and carbon, respectively. This precursor was dried at 70oC, and pyrolyzed at 400oC to obtain the blends of ZrO₂ and carbon, and at 1600oC to obtain ZrC by carbo-thermal reaction. Fourthly, ZrB₂ precursor was prepared with ZrOCl2·8H2O, boric acid and phenolic resin as sources of zirconium, boron and carbon, respectively. When dried at 40oC, the precursor turned into sol, then gel, and at last loose solid. The precursor decomposed into ZrO₂, B₂O3 and carbon under 400oC, and turned into ZrB₂ at 1600oC. The TiC precursor and hydroxyl-ZrC precursor had better cross-linking and pyrolysis properties, and met the requirements of PIP process.C/ZrC composite was fabricated by PIP process while the mixture of Zr(OBu)4 and DVB was used as ZrC precursor. A process of inorganic treatment at low temperature and carbo-thermal treatment at high temperature was established to improve the densification efficiency and the properties of the composite. The parameters were optimized as cross-linking at 150oC, inorganic treatment at 700oC, carbo-thermal treatment at 1600oC, and 20 PIP cycles. The as-obtained C/ZrC composite had a tensile strength of 253.6 MPa, elastic modulus of 42.3 GPa, and fracture toughness of 14.54 MPa·m1/2. After ablated by oxyacetylene flame for 300 seconds, the C/ZrC composite showed a mass loss rate of 0.0059 g/s and a linear recession rate of 0.0040 mm/s.CVI-SiC, PIP-SiC, PIP-C interface coatings were fabricated on carbon fiber by CVI and PIP processes, respectively. The mechanical properties of the C/ZrC composites with CVI-SiC interface coatings decreased, and those of the C/ZrC composites with PIP-SiC and PIP-C interface coatings increased. Furthermore, the CVI-SiC and PIP-SiC interface coatings improved the ablation properties of the C/ZrC composite. The SiCPIP2-C/ZrC composite with PIP-SiC interface coating of 2 cycles had the best properties: a tensile strength of 319.2 MPa, elastic modulus of 46.3 GPa, fracture toughness of 18.81 MPa·m1/2, mass loss rate of 0.0098 g/s and linear recession rate of 0.0089 mm/s during the oxyacetylene torch test.C/ZrC-SiC composite was fabricated to improve the oxidation property of the composite. After oxidized at 1200oC for 0.5 hour, the C/ZrC-SiC composite showed a tensile strength of 157.5 MPa and an elastic modulus of 22.0 GPa, which were much better than those of C/ZrC composite. The tensile strength of the C/ZrC-SiC composite was 322.0 MPa, the elastic modulus was 48.3 GPa, the fracture toughness was 11.55 MPa·m1/2. The mass loss rate and linear recession rate of the C/ZrC-SiC composite were 0.0089 g/s and 0.0136 mm/s in oxyacetylene flame environment, respectively, while these were 0.0181 g/s and 0.0037 mm/s in arc-jet wind tunnel environment, respectively. The SiO₂ melt layer on the ablation surface from the SiC oxidation increases the mass loss rate but decreases the linear recession rate of the C/ZrC-SiC composite. When ablated in arc-jet wind tunnel environment, the C/ZrC-SiC composite showed a worse ablation property than C/ZrC composite.The ablation mechanism of the C/ZrC composite in oxyacetylene flame environment was discussed. Thermal-chemical process was the main ablation mechanism during the oxyacetylene torch test. ZrC and carbon was oxidized into ZrO₂ and CO₂. After the ablation, the composite was divided into four layers as the melting layer, the loose tree-coral-like ZrO₂ layer, the undersurface oxidation layer and the composite layer. The melting layer was divided into melting, porous and marginal region. Though slight difficulties existed in the different regions, ZrO₂ melt existed in all regions and partially sealed the pores and protected the composite. The ablation process may be divided into two steps according to the temperature gradient. Firstly, oxidation of carbon fiber and ZrC matrix began at relative lower temperatures to form a porous structure. In this step the ablation rate was controlled by the oxidation rate of carbon and ZrC. Secondly, ZrO₂ melted and spread on the ablation surface while the temperature was raised. The ablation rate was controlled by the diffusion rate of oxygen through the ZrO₂ melting layer.The ablation mechanism of C/ZrC-SiC composite with PIP-SiC interface coating in oxyacetylene flame environment was investigated. The ablation of C/ZrC-SiC composite was a combination of thermal-chemical ablation and mechanical erosion. ZrC, SiC and carbon were oxidized into ZrO₂, SiO₂ and CO₂ respectively. The ablated surface was divided into a melting region, a SiO₂ depleted region and a SiO₂ enriched region. Through thickness the ablated composite was divided into a slurry layer, a grain-like SiO₂ depleted layer, a melting SiO₂ enriched layer and a composite layer. The melting SiO₂ enriched layer was dense and integral, which protected the composite. With temperature increase the ablation process of C/ZrC-SiC composite may be divided into three steps. Firstly, oxidation of carbon fiber, SiC and ZrC matrix began at relative lower temperature to form a porous structure. In this step the ablation rate was controlled by the oxidation rate. Secondly, while the temperature was raised, SiO₂ melted and spread on the ablated surface and was blown away by the gas flux. The ablation rate was controlled by the diffusion rate of oxygen through the SiO₂ melting layer and the eroding rate. Thirdly, SiO₂ evaporated after the temperature reached the boiling point of SiO₂. The ablation rate was controlled by the diffusion rate of oxygen, the evaporate rate of SiO₂ and the eroding rate.The ablation mechanisms of the ultra high temperature composites during arc-jet wind tunnel test were also investigated. For C/ZrC composite, dense ZrO₂ melting layer formed during the arc-jet wind tunnel test, adhered to the ablation surface firmly, and protected the composite. Thus, the C/ZrC composites showed an excellent ablation property, and chemical reaction was the main mechanism during the arc-jet wind tunnel test. When C/ZrC-SiC composite was ablated by arc-jet wind tunnel, low viscosity ZrO₂-SiO₂ melting layer formed and was blown away from the surface. The composite was exposed to the ablation flow and ablated seriously. Chemical reaction and mechanical erosion was the main mechanism of the C/ZrC-SiC composite during the arc-jet wind tunnel test.